1. Field of the Invention
A method used in a wireless communication system and related communication device are provided, and more particularly to, a method for reducing closed subscriber group (CSG) identity comparison in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A closed subscriber group (CSG) cell is accessible only for a limited set of UEs (i.e. closed subscriber group). In order to prevent a UE from attempting to register on a CSG cell on which they do not have access, the UE maintains an allowed CSG list, i.e. a list of CSG identities (ids) for which access has been granted to the UE. The CSG id is an identifier broadcast by the CSG cell or cells and used by the UE to facilitate access for authorized members of the associated closed subscriber group.
All the CSG cells sharing the same identity are identifiable as a single group for the purposes of mobility management and charging in a PLMN.
In order to avoid handover attempts to CSG cells which are not in the UE's Allowed CSG list and operator CSG List, a preliminary access check of the measured CSG cell needs to be performed by the UE. The network typically only initiates the handover preparation towards a reported CSG cell if the UE indicates that the reported cell is an allowed CSG cell for the UE. Preliminary access check is not necessary for hybrid cells. The hybrid cell is accessed as a CSG cell by a UE whose Allowed CSG List or Operator CSG List contains the cell's CSG identity and as a normal cell by all other UEs.
In autonomous and normal cell reselection case, the UE having a non-empty allowed CSG list looks for the CSG cell. A UE supporting both UTRAN and E-UTRAN access modes, is capable of camping on both UTRAN and E-UTRAN CSG cells. During the CSG cell selection/reselection procedure, the UE compares the CSG id of the cell with its allowed CSG list to check if the cell belongs to its CSG group or not. Currently, the allowed CSG list has no association of CSG ID with its RAT, i.e. there is no distinction between E-UTRAN and UTRAN CSG IDs. The UE compares the CSG id of the cell being evaluated for cell selection/reselection with all CSG IDs in the allowed CSG list. This includes unnecessary comparisons of CSG id of the cell being evaluated with CSG ids in stored list belonging to other RAT.
This unnecessary comparison delays the cell selection and reselection procedure and impacts battery usage. The same problem also happens in the preliminary check procedure when the UE compares the cells CSG IDs with the allowed CSG list during the measurement reporting. The delay in sending the measurement report due to unnecessary CSG IDs comparison may cause a handover delay or radio link failure.